A Field Study on Groyne Field Nourishments

Over the past century, the main channel of the Waal has experienced erosion of approx-imately 1-2 metres (Ylla Arb´os et al., 2021; Chowdhury et al., 2023). This erosion leads to various problems such as instability of struc-tures or disruption to shipping. To address this ongoing degradation, a potential solution is the implementation of...

A Well-Posed Alternative to the Hirano Active Layer Model for Rivers With Mixed-Size Sediment

The active layer model (Hirano, 1971) is frequently used for modeling mixed-size sediment river morphodynamic processes. It assumes that all the dynamics of the bed surface are captured by a homogeneous top layer that interacts with the flow. Although successful in reproducing a wide range of phenomena, it has two problems: (1) It may become...

Ill posedness in modelling two-dimensional morphodynamic problems: Effects of bed slope and secondary flow

A two-dimensional model describing river morphodynamic processes under mixed-size sediment conditions is analysed with respect to its well posedness. Well posedness guarantees the existence of a unique solution continuously depending on the problem data. When a model becomes ill posed, infinitesimal perturbations to a solution grow infinitely...

A reduced complexity model of a gravel-sand river bifurcation: Equilibrium states and their stability

We derive an idealized model of a gravel-sand river bifurcation and analyze its stability properties. The model requires nodal point relations that describe the ratio of the supply of gravel and sand to the two downstream branches. The model predicts changes in bed elevation and bed surface gravel content in the two bifurcates under...

Ill-posedness in modeling mixed sediment river morphodynamics

In this paper we analyze the Hirano active layer model used in mixed sediment river morphodynamics concerning its ill-posedness. Ill-posedness causes the solution to be unstable to short-wave perturbations. This implies that the solution presents spurious oscillations, the amplitude of which depends on the domain discretization. Ill-posedness...

A Sand-Gravel Gilbert Delta Subject to Base Level Change

Laboratory experiments were conducted on a sand-gravel Gilbert delta to gain insight on its dynamics under varying base level. Base level rise results in intensified aggradation over the topset, as well as a decrease in topset slope and topset surface coarsening, the signals of which migrate in an upstream direction. Preferential deposition...

Advance, Retreat, and Halt of Abrupt Gravel-Sand Transitions in Alluvial Rivers

Downstream fining of bed sediment in alluvial rivers is usually gradual, but often an abrupt decrease in characteristic grain size occurs from about 10 to 1 mm, i.e., a gravel-sand transition (GST) or gravel front. Here we present an analytical model of GST migration that explicitly accounts for gravel and sand transport and deposition in the...

The equilibrium alluvial river under variable flow and its channel-forming discharge

An accurate numerical solution to the Saint-Venant-Hirano model for mixed-sediment morphodynamics in rivers

We present an accurate numerical approximation to the Saint-Venant-Hirano model for mixed-sediment morphodynamics in one space dimension. Our solution procedure originates from the fully-unsteady matrix-vector formulation developed in [54]. The principal part of the problem is solved by an explicit Finite Volume upwind method of the path...

A laboratory experiment on the evolution of a sand gravel reach under a lack of sediment supply

A flume experiment was conducted to examine the evolution of a sand-gravel reach under a lack of sediment supply. A bed composed of a bimodal sediment mixture was installed with a uniform slope and an gradual fining pattern. At the upstream end of the flume the initial bed consisted of 100% gravel, which decreased stepwise (10% steps) in...

Mathematical analysis of the Saint-Venant-Hirano model for mixed-sediment morphodynamics

Sediment of different size are transported in rivers under the action of flow. The first and still most popular sediment continuity model able to deal with mixed sediment is the so-called active layer model proposed by Hirano (1971, 1972). In this paper, we consider the one-dimensional hydromorphodynamic model given by the Saint-Venant equations...

The effect of river dunes on the morphodynamic response to overloading

A new two-layer model for conservation of mixed sediment for bedform-dominated condi-tions is proposed. There is a need for such a new bedlayer-type sediment conservation model, as (i) the commonly applied Hirano (1971) model does not account for the effects of bedform stochastics and ver-tical sorting on bed level changes, (ii) the Ribberink...

Different approaches to handling vertical and streamwise sorting in modeling river morphodynamics

This paper presents an assessment of the strengths and weaknesses of four sediment continuity models for nonuniform sediment by applying these models to an aggradational flume experiment that is dominated by nonuniform sediment and dunes. The author makes simulations of the flume experiment using four numerical morphodynamic model systems to...

A Vertical Sorting Model for Rivers with Non-uniform Sediment and Dunes

Vertical sorting fluxes and the resulting organisation of grain size fractions over bed elevations, i.e. the vertical sorting profile, influence bedform dimensions and the bed surface composition. As a result, they will affect the bed roughness, the rate and composition of the sediment transport, the large-scale river morphology, and water...